The present invention relates to a rotation angle sensor which detects a rotation angle of a rotation shaft in such a manner that a hall element detects a magnetic field having intensity gradually changing in the circumferential direction of a disk of a yoke rotating together with the rotation shaft.
The following Patent Document 1 discloses a rotation angle sensor which detects a rotation angle of a rotation shaft in such a manner that a hall element detects a magnetic field having intensity gradually changing in the circumferential direction of a disk of a yoke rotating together with the rotation shaft.
FIGS. 7 and 8 show a configuration of the rotation angle sensor having the same structure as that of the rotation angle sensor disclosed in the following Patent Document 1 and detecting the rotation of the rotation shaft.
A rotation angle sensor 1 shown in FIGS. 7 and 8 includes a yoke 7 which is made from a highly permeable material and is provided in a rotation shaft 5 rotatably supported to a sensor casing 3, a permanent magnet 9 which is fixed to the yoke 7, and a hall element (hall IC) 11 which generates an output signal in response to a force of an applied magnetic field.
Since the rotation shaft 5 has a shaft engagement hole 5a which is formed through the center of the rotation shaft and engages with a driving shaft (not shown), the rotation shaft rotates together with the driving shaft engaging with the shaft engagement hole 5a. 
As shown in FIGS. 7, 10A and 10B, the yoke 7 has a configuration in which a pair of disks 7a and 7b fixed to two positions of the rotation shaft 5 in a flange shape and spaced from each other in the axial direction of the rotation shaft 5 is integrally formed with a connection portion 7c magnetically coupling the pair of disks 7a and 7b through press molding using a highly permeable metal plate.
The pair of disks 7a and 7b is attached to the rotation shaft 5 so as to be perpendicular to the axial direction of the rotation shaft 5. A shaft engagement hole 8 is formed through each of the disks 7a and 7b so as to be concentric with the shaft engagement hole 5a. 
As shown in FIG. 9, the permanent magnet 9 includes a pair of magnet plates 9a and 9b respectively laminated on the facing surfaces of the pair of disks 7a and 7b. 
The pair of magnet plates 9a and 9b has substantially the same shape as those of the pair of disks 7a and 7b in a plan view. As shown in FIGS. 9, 11A and 11B, a shaft engagement hole 10 is formed through the center of each of the magnet plates so as to be concentric with the shaft engagement hole 5a. 
As for the shape from a side view, the pair of magnet plates 9a and 9b has a taper structure in which a plate thickness gradually changes in the circumferential direction as shown in FIG. 11A. This is because a magnetic field is formed so as to have a magnetic force gradually changing in the circumferential direction along the outer peripheries of the pair of disks 7a and 7b. 
The rotation shaft 5 is made from a resin. The pair of disks 7a and 7b and the magnet plates 9a and 9b are integrally formed with the rotation shaft 5 through insert molding, and a shaft assembly 15 shown in FIG. 9 can be assembled for the sensor casing 3.
The hall element 11 is fixed to the sensor casing 3 so that a magnetic field detecting portion 11a is located in the middle of a gap between the pair of permanent magnets 9 and 9 facing each other in the outer peripheries of the pair of disks 7a and 7b. The output terminal of the hall element 11 is connected to an external connection terminal 17 installed in the sensor casing 3.
In the rotation angle sensor 1 shown in FIGS. 7 and 8, as described above, the magnet plates 9a and 9b have a structure in which the thickness of each of the magnet plates gradually changes in the circumferential direction of the pair of disks 7a and 7b. Accordingly, when the shaft assembly 15 is rotationally driven in a direction depicted by the arrow E of FIG. 8, a spaced distance (gap) between the magnetic field detecting portion 11a and each of the magnet plates 9a and 9b gradually changes in the circumferential direction of the disks 7a and 7b. Accordingly, since the magnetic flux density crossing the magnetic field detecting portion 11a changes, the output of the hall element 11 changes in response to the rotation angle of the shaft assembly 15, thereby detecting the rotation angle of the rotation shaft 5.    [Patent Document 1] JP-A-2000-028312
However, in the related rotation angle sensor 1 having a structure in which a plate thickness of each of the magnet plates 9a and 9b fixed to the disks 7a and 7b gradually changes as shown in FIG. 11A in order to form a magnetic field having intensity gradually changing in the circumferential direction along the outer peripheries of the pair of disks 7a and 7b, since the weight increases due to the increased plate thickness of each of the magnet plates 9a and 9b, and trouble is taken for the operation of processing the magnet plates 9a and 9b each having a tapered surface, there is a problem in that it is difficult to decrease the cost.